1. Field of the Art
The present invention relates to a display system for attaining a desired display pattern by applying an electric field between opposed electrodes, and more particularly to a display apparatus having a display panel with a novel electrode pattern and means for driving the display panel.
2. Description of the Prior Art
As display systems for achieving display by applying an electric field between opposed electrodes, various types of display systems are known such as those utilizing liquid crystals, those making use of gas discharge and those employing electroluminescence. In the following description, they will be explained, by way of example, in connection with liquid crystal display systems which have been widely popularized and developed even for color displays. In field of the color liquid crystal display systems, a guest-host type liquid display panel making use of a cholesteric-nematic mixed liquid crystal added with a pleochroic dye, is known as reported by D. L. White and G. N. Taylor in "Journal of Applied Physics, Vol. 45 (1974) pp. 4718-4723." This guest-host type liquid crystal display panel can work without an expensive polarizer which is necessary for a color liquid crystal display utilizing other electro-optical effects. Accordingly, not only is the display panel cheap, but it also has the advantage that the display becomes brighter and the visible angle becomes as wide as 140 degrees or more of a solid angle. The principle of operation of a guest-host type liquid crystal display is described as follows.
The surface of an electrode substrate making contact with liquid crystal molecules is subjected to a homeotropic surface treatment so that upon application of no voltage the liquid crystal molecules in the proximity of the electrode substrate may be oriented perpendicular to the substrate surface. Then the liquid crystal molecules in the liquid crystal bulk are oriented in a spiral manner, and the axis of the spiral is perpendicular to the substrate surface. The optical axes of the pleochroic dye molecules are likewise oriented in a spiral manner along the spiral orientation of the liquid crystal molecules in the bulk. Accordingly, among the incident light, the light component in the wavelength range inherent to the added pleochroic dye is absorbed, and hence the liquid crystal looks colored for an observer. When a voltage difference higher than the threshold voltage or the transition voltage of the liquid crystal is applied between the opposed electrodes, phase change occurs from a cholesteric phase to a nematic phase, so that the liquid crystal molecules take the orientation perpendicular to the electrode substrate, that is, the homeotropic alignment. Accordingly, the optical axes of the pleochroic dye molecules also become perpendicular to the electrode substrate, and thus since the direction of propagation of the incident light becomes parallel to the optical axes of the pleochroric die molecules, the liquid crystal looks to have the color of a reflecting plate placed on the backside of the panel, for instance, to be white for an observer.
As will be apparent from the above description, the common mode of use of the above-described type of color liquid crystal display panels is display of white characters on a colored background (hereinafter called "negative display"), because the character region is decolored by the presence of an applied field. While such a color display panel has the advantages of a very wide viewing angle, sufficient brightness and being fashionable, the display of colored characters on a white background (hereinafter called a "positive display") would be preferable to a negative display in view of human perception. In order to realize a positive display, a transparent electrode pattern must be designed so that a transition voltage can be applied across the background region of characters. In general, a display area of a display panel consists of unit display areas and background area. Each of the unit display areas can simply be called a segment. As well known, the Arabic numeral 0 to 9 can be shown by making use of seven segments, which are arrayed in a figure eight and which are different from the background area. In the conventional negative display, wanted segments needed for a desired numeral are energized while the background area remains unenergized. In the positive display, however, the background area and unwanted segments are energized while the wanted segments for display remain unenergized.
For a positive display, therefore, the electrode patterns on the front and back substrates must be different from the conventional negative display panel. In a positive display panel of a single digit, for instance, a primitive electrode patters can be thought of as follows. An electrode layer formed on a front substrate is divided into nine sections to provide nine individual electrodes. While seven electrodes are arrayed so as to selectively energize unwanted segments and its outside background, the remaining two electrodes are arrayed to energize only inside backgrounds of the figure eight. On the other hand, a back electrode substrate has eight divided electrodes closely arrayed to each other, and one of which is a so called common electrode to be used for energizing unwanted segments and two inside backgrounds. The common electrode is designed so as to face all segments and two front electrodes used only for the inside backgrounds. The remaining seven electrodes are arrayed so that each electrode faces each seven front electrodes except for the segment areas.
With this positive display panel, a desired numeral can be obtained by selectively applying the transition voltage between front and back electrodes. However, with regard to the number of channels of a driving circuits, 16 channels in total are required. This number is twice as many as the number of channels in the case of the conventional negative display, hence the driving circuit becomes expensive and the driving operation becomes complex.